Fault current limiter

ABSTRACT

A fault current limiter (FCL) includes a series of high permeability posts for collectively define a core for the FCL. A DC coil, for the purposes of saturating a portion of the high permeability posts, surrounds the complete structure outside of an enclosure in the form of a vessel. The vessel contains a dielectric insulation medium. AC coils, for transporting AC current, are wound on insulating formers and electrically interconnected to each other in a manner such that the senses of the magnetic field produced by each AC coil in the corresponding high permeability core are opposing. There are insulation barriers between phases to improve dielectric withstand properties of the dielectric medium.

PRIORITY CLAIM

The present application is a Continuation of U.S. patent applicationSer. No. 12/866,321 filed on Aug. 5, 2010 now U.S. Pat. No. 8,027,135;which is a National Phase Application of PCT Application Serial No.PCT/AU2009/000409 filed on Apr. 3, 2009 which claims the benefit of AUPatent Application Serial No. 2009 901138 filed on Mar. 16, 2009 and AUPatent Application Serial No. 2008 901584 filed on Apr. 3, 2008, thedisclosure of these applications is hereby incorporated herein byreference.

GOVERNMENT RIGHTS

The United States Government has certain rights in the inventionpursuant to a contract with the U.S. Department of Energy.

FIELD OF THE INVENTION

The present invention relates to a fault current limiter.

The invention has been developed primarily for a high voltage saturatedcore fault current limiter and will be described with reference to thatapplication. However, the invention is not limited to that particularfield of use and is also suitable for low voltage, medium voltage,extra-high voltage and ultra-high voltage fault current limiters.

BACKGROUND OF THE INVENTION

Saturated core fault current limiters (FCLs) are known. Examples ofsuperconducting fault current limiting devices include:

-   U.S. Pat. No. 7,193,825 to Darmann et al.-   U.S. Pat. No. 6,809,910 to Yuan et al.-   U.S. Pat. No. 7,193,825 to Boenig,-   US Patent Application Publication Number 2002/0018327 to Walker et    al.

The fault current limiters described are for use with dry insulationtype copper coil arrangements and, in practical terms, only suitable forDC saturated FCLs which employ air as the main insulation medium. Thatis, the main static insulation medium between the AC phase coils in apolyphase FCL and between the AC phase coils and the steel core, DCcoil, cryostat, and main structure is provided by a suitable distance inair. This substantially limits the FCL to a “dry type” insulationtechnologies. Dry type technologies normally refers to those transformerconstruction techniques which employ electrically insulated copper coilsbut only normal static air and isolated solid insulation harriermaterials as the balance of the insulation medium. In general, air formsthe majority of the electrical insulation material between the highvoltage side and the grounded components of the FCL. These groundedcomponents include the steel frame work and the case.

The utilisation of dry type insulation limits the FCL to lower voltageranges of AC line voltages of up to approximately 39 kV. Dry typetransformers and reactors are only commercially available up to voltagelevels of about 39 kV. As a result, the current demonstrated technologyfor DC saturated FCL's is not suitable for extension into high voltageversions. Dry type designs result in an inability to design apractically sized compact structure using air as an insulation mediumwhen dealing with higher voltages.

One of the main emerging markets for FCL's is the medium to high voltage(33 kV to 166 kV) and extra-high voltage range (166 kV to 750 kV). Whenoperating within these voltage ranges, the currently described art andliterature descriptions of DC saturated FCL's are not practical. Themain reason is due to static voltage design considerations—for example,the breakdown of the air insulation medium between the high voltagecopper coils and the cryostat or steel core or DC coil. High voltagephase coils at medium to high voltages (greater than 39 kV) often needto be immersed in one of

-   -   An insulating gas (such as SF₆, nitrogen, or the like).    -   A vacuum (better than 10⁻³ mbar).    -   A liquid such as a synthetic silicone oil, vegetable oil, or        other commonly available insulating oils used in medium, high        voltage, and extra-high voltage transformer and reactor        technology.

When a high voltage device is immersed in such an insulating medium,that medium is often referred to as the “bulk insulation medium” or the“dielectric”.

Typically, the dielectric will have a relative permittivity of the orderof about 2 to 4, except for a vacuum which has a relative permittivityequal to 1. These so called dielectric insulation media haveelectrostatic breakdown strength properties which are far superior tothat of atmospheric air if employed judiciously by limiting the maximumdistance between solid insulation barriers and optimising the filleddielectric distance with respect to the breakdown properties of theparticular liquid or gaseous dielectric.

The commonly available bulk insulating gases and liquids typically havea breakdown strength in the order of 10 to 20 kV/mm but are usuallyemployed such that the average electric field stress does not exceedabout 6 to 10 kV/mm. This safety margin to the breakdown stress value isrequired because even if the average electrostatic field stress is 6 to10 kV/mm, the peak electrostatic field stress along any isostaticelectric field line may be 2 to 3 times the average due to variouselectrostatic field enhancement effects.

In general, there are five main desirable requirements of a dielectricliquid or gas for high voltage bulk insulation requirements in housedplant such as transformers and reactors and fault current limiters:

-   -   The dielectric must show a very high resistivity.    -   The dielectric losses must be very low.    -   The liquid must be able to accommodate solid insulators without        degrading that solid insulation (for example, turn to turn        insulation on coil windings or epoxy).    -   The electrical breakdown strength must be high,    -   The medium must be able to remove thermal energy losses.

Solid insulation techniques are not yet commonly available at medium tohigh voltages (that is, at operating voltages greater than 39 kV) forhoused devices such as transformers, reactors and fault currentlimiters. The shortcoming of solid insulation techniques is the presenceof the inevitable voids within the bulk of the solid insulation orbetween surfaces of dissimilar materials such as between coil insulationand other solid insulation materials. It is well known that voids insolid insulation with high voltages produce a high electric stresswithin the void due the field enhancement effect. This causes physicalbreakdown of the surrounding material due to partial discharges and caneventually lead to tracking and complete device failure.

It will be recognized that a DC saturated belt current limiter whichemploys a single or multiple DC coils for saturating the steel core,such as those disclosed in the aforementioned prior art, posesfundamental problems when the copper AC phase coils can no longer be ofa “dry type” construction or when the main insulation medium of thecomplete device is air. A significant problem in such arrangements isthe presence of the steel cryostat for cooling the DC HTS coil and theDC HTS coil itself. The cryostat and the coil and the steel cores areessentially at ground potential with respect to the AC phase coils.

As a side issue, but one which enhances the insulation requirements forall high voltage plant and equipment, it is that basic insulation designmust also meet certain electrical engineering standards which test fortolerance to various types of over-voltages and lighting impulses overpredetermined time periods. An example, in Australia, of such standardsare as follows:

-   -   AS2374 Part3. Insulation levels and dielectric tests which        includes the power frequency (PF) and lightning impulse (LI)        tests of the complete transformer.    -   AS2374 Part 3.1. Insulation levels and dielectric tests—External        clearances in air.    -   AS2374 Part 5. Ability to withstand short-circuit.

These standards do not form an exhaustive list of the standards thathigh voltage electric equipment must meet. It is recognised that eachcountry has their own standards which cover these same design areas andreference to an individual country's standard does not necessarilyexclude any other country's standards. Ideally a device is constructedto meet multiple countries standards.

Adherence to these standards result in a BIL (Basic Insulation level)for the device or a “DIL” (Design Insulation Level) which is usually amultiple of the basic AC line voltage. For example, a 66 kV mediumvoltage transformer or other housed device such as a FCL may have a BILof 220 kV. The requirement to meet this standard results in a staticvoltage design which is more strenuous to meet practically than from aconsideration of the AC line voltage only. The applicable standards andthis requirement has resulted from the fact that a practical electricalinstallation experiences temporary over voltages which plant and devicesmay experience within a complex network, for example lightning overvoltages, and switching surges. Hence, all equipment on an electricalnetwork has a BIL or DIL appropriate for the expected worst casetransient voltages.

An initial consideration of the static design problem for high voltageDC saturated fault current limiters may result in the conclusion thatthe problem is easily solved by housing only the high voltage AC coppercoils in a suitable electrical insulating gas or liquid. However, theproblem with this technique is that the steel core must pass through thecontainer which holds the gas or liquid. Designing this interface forlong term service is difficult to solve mechanically. However, moreimportantly solving the interface problem electrostatically is much morecomplex and any solution can be prone to failure or prove uneconomical.The problem is that as a seal must be developed between the vesselcontaining the dielectric fluid and the high permeability core or,alternatively, a method of isolating the HTS cryostat from the fluid.

Another possibility is the use of solid high voltage barriers betweenphases and between phases and the steel core and cryostat or a layer ofhigh voltage insulation around the copper phase coils and in intimatecontact with the phase coils. However, this has a significantdeleterious side effect. It is known that the static electric field in acombination of air and other materials with a higher relativepermittivity is that this always results in an enhanced electric fieldin the material or fluid with the lower permittivity (that is air). Forexample, consider a conductive copper cylinder with a layer of normalinsulation to represent the turn to turn insulation, according equation1:

= U m x ⁢ { ln ⁡ [ R r ] + ln ⁡ [ d R ] 1 } ⁢ Equation ⁢ ⁢ 1where:

-   -   U_(m)=AC phase voltage with respect to ground.    -   R=radius of a copper cylinder including outside insulation [mm].    -   R=radius of bare copper cylinder [mm],    -   D=distance from center of cylinder to the nearest ground plane        [mm].    -   ∈₂=relative dielectric constant of the insulation covering the        cylinder    -   ∈₁=relative dielectric constant of the bulk insulation where the        cylinder is immersed (which equals 1 for air),    -   x=distance from the center of cylinder to a point outside the        cylinder [mm],    -   E_(x)=Electrostatic field gradient at point x[kV/mm].

The field enhancement effect is represented by the factor ∈₂/∈₁ and isof the order 2 to 4 for common everyday materials except for the case ofemploying a vacuum which has a relative permittivity equal to 1. Byproviding additional solid or other insulation material (of higherelectric permittivity than air) there is an increase in theelectrostatic stress in the bulk air insulation of the FCL. The betterthe quality of the high voltage insulation, the higher the fieldenhancement effect.

Hence, solid dielectric insulation barriers in an otherwise airinsulated FCL are not a technically desirable option for high voltageFCL's at greater than 39 kV and indeed one does not see this techniquebeing employed to make high voltage dry type transformers at greaterthan 39 kV for example. In fact, no techniques have been found highlysuitable to date and that is why high voltage transformers above 39 kVare insulated with a dielectric liquid or gas.

The discussion above is the reason why housed high voltage electricalequipment is often completely immersed in electrically insulatingdielectric fluid or gas. That is, the insulated copper coils and thesteel core of transformers and reactors are housed within a containerthat is then completely filled with a dielectric medium which is afluid. This substantially reduces the electrostatic voltage designproblems detailed in the above discussion. The insulating medium (forexample oil, vacuum, or SF₆) fills all of the voids and bulk distancesbetween the high voltage components and the components which areessentially at ground or neutral potential. In this case, solidinsulation barriers may be incorporated into the bulk insulatingdielectric and for many liquids such as oil, dividing the largedistances with solid insulation improves the quality of the overallelectrostatic insulation by increasing the breakdown field strength ofthe dielectric fluid. This is because the relative permittivity of theoil and solid insulation are very close to each other (so fieldenhancement effects are lessened compare to air) and the breakdownvoltage of the bulk dielectric medium (expressed in kV/mm) improves forsmaller distances between the insulation barriers.

A major problem with the full immersion technique is that it is notreadily adaptable to a DC saturated FCL designs or other devices thatincorporated a superconductor coil as the DC saturating element. This isbecause the superconducting coil and its cryostat or vacuum vessel are acomponent of the FCL which must also necessarily be immersed in thedielectric fluid.

The established body of literature clearly points to four main criteriafor a marketable, feasible, and manufacturable FCL:

-   -   It must have a low insertion impedance so that it is invisible        to the network when there are no faults and when providing peak        power flow.    -   It must not produce more than 0.5% THD worth of harmonics (Total        harmonic distortion) or as required by the end user.    -   It must provide a suitable clip of the fault current, between 20        to 80%.    -   The design must be augmentable to high AC voltages (greater than        6 kV) and high AC current (greater than 0.6 kA).

The classic saturable core FCL designs detailed in the prior art sufferthe major drawbacks of not being suitable for high voltage and high ACcurrent designs. Both of these disadvantages originate from the lack ofa coolant (other than air) and/or a liquid or gaseous dielectric.

Even if a liquid or gaseous dielectric is employed in the classicsaturable FCL design, there is still required significant augmentationto allow access to the cryocooler, cryostat, and cryostat fittings. Inaddition, special seals to isolate the cryostat feed-throughs(electrical power, electrical signals) from the dielectric have to bemade and tested.

In high AC current designs, the cross sectional area of copper requiredto conduct the required electrical current is much higher whenconsidering only an air cooled design. It is not unusual for this crosssection area to be up to five times higher. This can make the dimensionsof the AC coil too large to be accommodated into the minimum core frameyoke size, requiring a larger yoke to maintain electrostatic clearance.This increases the footprint and mass of the classic air cooled/airinsulated saturable FCL.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

SUMMARY OF THE INVENTION

It is an object of the preferred embodiments of this invention toameliorate one or more of the aforementioned disadvantages or to providea useful alternative.

It is another object of the preferred embodiments of the invention toovercome one or more of the above-stated disadvantages by inverting theconventional relative locations of the AC and DC coils with an FCL.These embodiments allow the complete structure to be immersed in adielectric.

According to a first aspect of the invention there is provided a highvoltage fault current limiter which includes a magnetically saturablecore and at least one AC phase coil wound around a portion of saidsaturable core wherein said magnetically saturable core and said atleast one AC phase coil are housed within an enclosure and a DC biasingcoil is disposed outside of and surrounding said enclosure which duringno fault operating conditions of said current limiter biases said coreinto magnetic saturation for low steady state un-faulted insertionimpedance but during fault conditions takes said core out of magneticsaturation to thereby provide an increased current limiting impedance insaid electrical circuit.

In an embodiment, the high permeability core is selected from one ormore of a transformer steel lamination material; a mild steel; or otherforms of magnetic steel, ferrite materials or a ferromagnetic material.

In an embodiment, the core is in the form of a rectangular array of coreposts with AC phase coils wound one each on respective ones of the coreposts and electrically interconnected in a manner such that the sensesof the magnet fields produced by the AC coils are opposing.

In an embodiment, the fault current limiter includes a vesselsurrounding the AC coils for containing a dielectric insulation mediumand cooling medium for said AC coils.

In an embodiment, the DC coil is a superconductor and more preferably ahigh temperature superconductor housed in a cryostat and cooled by acryocooler.

In an embodiment, the DC biasing coil is coincident with a coaxial withthe AC phase coils so that said portion of the saturable core is fullysaturated.

In an embodiment, the magnetically saturable core and AC coils areimmersed in a dielectric which is in the form of a solid, liquid or gasand including air at any atmosphere including vacuum.

In an embodiment, the core posts are rectangular in cross-section and ofconstant cross-section along the lengths thereof.

In an embodiment, the magnetically saturable core is constructed from atransformer steel lamination material, mild steel or other magneticsteel, ferrite material, an insulated high permeability compressedpowder, or a ferromagnetic material.

In an embodiment, the core posts are tapered toward the ends thereofwhereby during no fault operation of the current limiter substantiallyall of said core is saturated.

According to a second aspect of the invention there is provided a faultcurrent limiter including:

an input terminal for electrically connecting to a power source thatprovides a load current;

an output terminal for electrically connecting with a load circuit thatdraws the load current;

a magnetically saturable core;

an AC coil wound about a longitudinal portion of the core for carryingthe load current between the input terminal and the output terminal; and

at least one DC coil for inducing a magnetic field in at least theportion of the core and extending about a longitudinal intermediate zonethat receives the core and the AC coil, wherein the field magneticallybiases the core such that the AC coif moves from a low impedance stateto a high impedance state in response to one or more characteristics ofthe load current.

In an embodiment, in the low impedance state, the portion ismagnetically saturated.

In an embodiment, in the low impedance state, the core is magneticallysaturated longitudinally beyond the portion.

In an embodiment, in the high impedance state, the portion is out ofmagnetic saturation.

In an embodiment, in the low impedance state, the impedance of the ACcoil is substantially equal to the theoretical air core impedance of theAC coil.

In an embodiment, one of the one or more characteristics is an increaseof the load current beyond a predetermined current value.

In an embodiment;

the core includes a plurality of posts;

the longitudinal portion is segmented between the posts; and

the AC coil includes a plurality of coil segments that are wound aboutrespective posts.

In an embodiment, the posts are parallel

In an embodiment, the posts extend longitudinally.

In an embodiment, each post has a substantially uniform transversecross-section.

In an embodiment, the posts have substantially like transversecross-sections.

In an embodiment, the transverse cross-section of the posts has at leastone axis of symmetry.

In an embodiment, the transverse cross-sections of the posts aresymmetric.

In an embodiment, the posts substantially co-extend within theintermediate zone.

In an embodiment, the posts are spaced apart from each other.

In an embodiment, the posts extend longitudinally beyond the DC coils.

In an embodiment, the coil segments substantially longitudinallycoextend in the intermediate zone.

In an embodiment, the AC coil extends longitudinally beyond the DCcoils.

In an embodiment, each post extends longitudinally beyond the respectiveAC coil.

In an embodiment, the load current includes three phases and the faultcurrent limiter includes three pairs of input terminals and outputterminals for the respective phases.

In an embodiment, the fault current limiter includes six posts arrangedin three pairs, where each pair of posts is associated with a respectivepair of input and output terminals for carrying the corresponding phaseof the load current.

In an embodiment, the posts in each pairs of posts are yoked together.

In an embodiment, each post includes longitudinal ends, and at least oneend of each posts is yoked to an adjacent end of the other post in thesame pair.

In an embodiment, both ends of each posts are yoked to respectiveadjacent ends of the other post in the same pair.

In an embodiment, the posts are yoked magnetically and physically by ahigh permeability material.

In an embodiment, the posts in each pair are adjacent each other andinclude spaced apart opposing faces.

In an embodiment, the opposing faces are substantially planar.

In an embodiment, the opposing faces are substantially parallel.

In an embodiment, the opposing faces are substantially coextensive.

In an embodiment, the fault current limiter includes an enclosure fordefining the intermediate zone.

In an embodiment, the enclosure contains a dielectric material.

In an embodiment, the AC coil is received within the dielectric.

In an embodiment, the DC coils each include a high conductivitymaterial.

In an embodiment, the high conductivity material is selected from:copper; aluminium; a high temperature superconductive material; a lowtemperature superconductive material.

According to a third aspect of the invention there is provided a methodof limiting current including the steps of:

providing an input terminal for electrically connecting to a powersource that provides a load current;

providing an output terminal for electrically connecting to a loadcircuit that draws the load current;

providing a magnetically saturable core;

winding an AC coil about a longitudinal portion of the core for carryingthe load current between the input terminal and the output terminal; and

inducing a magnetic field in at least the portion of the core with atleast one DC coil, wherein the DC coil extends about a longitudinalintermediate zone that receives the core and the AC coil, and whereinthe field magnetically biases the core such that the AC coil moves froma low impedance state to a high impedance state in response to one ormore characteristics of the load current.

According to a fourth aspect of the invention there is provided a faultcurrent limiter including:

an input terminal for electrically connecting to a power source thatprovides a load current;

an output terminal for electrically connecting with a load circuit thatdraws the load current;

a magnetically saturable core;

an AC coil wound about a longitudinal portion of the core for carryingthe load current between the input terminal and the output terminal; and

at least one DC coil that is in an open-core arrangement with the ACcoil for inducing a magnetic field in at least the portion of the core,the DC coil extending about a longitudinal intermediate zone thatreceives the core and the AC coil, wherein the field magnetically biasesthe core such that the AC coil moves from a low impedance state to ahigh impedance state in response to one or more characteristics of theload current.

According to a fifth aspect of the invention there is provided a methodof limiting current using a fault current limiter, the method including:

electrically connecting a power source to an input terminal forproviding a load current;

electrically connecting a load circuit to an output terminal for drawingthe load current;

providing a magnetically saturable core;

providing an AC coil wound about a longitudinal portion of the core forcarrying the load current between the input terminal and the outputterminal; and

providing at least one DC coil that is in an open-core arrangement withthe AC coil for inducing a magnetic field in at least the portion of thecore, the DC coil extending about a longitudinal intermediate zone thatreceives the core and the AC coil, wherein the field magnetically biasesthe core such that the AC coil moves from a low impedance state to ahigh impedance state in response to one or more characteristics of theload current.

According to a sixth aspect of the invention there is provided a faultcurrent limiter including:

three input terminals for electrically connecting to respective phasesof a three phase power source that provides a three phase load current;

three output terminals for electrically connecting with the respectivephases of a load circuit that draws the load current;

a magnetically saturable core having three pairs of posts, each posthaving a longitudinal portion;

three AC coils wound about the portions of respective pairs of posts forcarrying the load current between the input terminals and the outputterminals; and

at least one DC coil for inducing a magnetic field in at least theportions and extending about a longitudinal intermediate zone thatreceives the posts and the AC coils, wherein the field magneticallybiases the core such that the AC coil moves from a low impedance stateto a high impedance state in response to one or more characteristics ofthe load current.

In an embodiment, each AC coil includes two coil segments that are eachwound about respective portions of the posts in the pair of posts.

According to a seventh aspect of the invention there is provided amethod of limiting current using a fault current limiter, the methodincluding the steps of:

electrically connecting to respective phases of a three phase powersource three input terminals for providing a three phase load current;

electrically connecting with the respective phases of a load circuitthree output terminals for drawing the load current;

providing a magnetically saturable core having three pairs of posts,each post having a longitudinal portion;

providing three AC coils wound about the portions of respective pairs ofposts for carrying the load current between the input terminals and theoutput terminals; and

providing at least one DC coil for inducing a magnetic field in at leastthe portions and extending about a longitudinal intermediate zone thatreceives the posts and the AC coils, wherein the field magneticallybiases the core such that the AC coil moves from a low impedance stateto a high impedance state in response to one or more characteristics ofthe load current.

According to an eighth aspect of the invention there is provided corefor a fault current limiter, the core including at least onelongitudinally extending post having at least two portions that aremagnetically saturable and which, in use, are received within respectivecoil segments of an AC coil that, in turn, is received within a DC coil.

In an embodiment, the portions are spaced apart.

In an embodiment, the cure includes two like parallel posts havingrespective portions.

In an embodiment, the posts are yoked.

In an embodiment, the posts are yoked to each other.

In an embodiment, each post extends between a first end and a secondend, wherein the first end and second end of one of the posts areadjacent to the first end and the second respectively of the other post.

In an embodiment, the core includes a yoke for extending between thefirst ends for yoking the posts to each other.

In an embodiment, the core includes a further yoke for extending betweenthe second ends for yoking the posts to each other.

In an embodiment, the posts include post laminations.

In an embodiment, the yokes include yoke laminations.

In an embodiment, the post laminations and the yoke laminations areinterleaved.

In an embodiment, the core includes six longitudinally extending postsarranged in three pairs.

According to a ninth aspect of the invention there is provided a faultcurrent limiter including a core of the eighth aspect of the invention.

According to a tenth aspect of the invention there is provided anelectrical distribution system including at least one fault currentlimiter of one of the first, second, fourth, sixth and ninth aspects ofthe invention.

Reference throughout this specification to “one embodiment”, “someembodiments” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention. Thus,appearances of the phrases “in one embodiment”, “in some embodiments” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment, but may.Furthermore, the particular features, structures or characteristics maybe combined in any suitable manner, as would be apparent to one ofordinary skill in the art from this disclosure, in one or moreembodiments.

As used herein, unless otherwise specified the use of the ordinaladjectives “first”, “second”, “third”, etc., to describe a commonobject, merely indicate that different instances of like objects arebeing referred to, and are not intended to imply that the objects sodescribed must be in a given sequence, either temporally, spatially, inranking, or in any other manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Currently preferred embodiments of the invention will now be describedwith reference to the following attached drawings in which:

FIG. 1 is a schematic view of a an experimental FCL core structure;

FIG. 2 illustrates the results of an FEA analysis on the structure ofFIG. 1;

FIG. 3 illustrates a closed core structure for an FCL with the AC coiland DC coil being overlayed and coaxial—that is, the two coils are woundabout the same limb of the closed core;

FIG. 4 illustrates an experimental closed core structure with associatedsearch coils for allowing an investigation of the nature of insertionimpedance;

FIG. 5 is an illustration of the results of the experiment conductedwith the structure of FIG. 4;

FIG. 6 summarises the measured insertion impedance results for the aboveexperimental structures;

FIG. 7 is a schematic cross-sectional view of a three phase open corefault current limiter according to said invention;

FIG. 8 is a schematic view of the electrical interconnection of thewindings on two of the core posts shown in the fault current limiter ofFIG. 7;

FIG. 9 shows FEA analysis results of the magnetic field and relativepermeability across the length of a core in the Z direction of FIG. 7;

FIG. 10 shows a plot of the magnetic field along a line central to thecores and crossing three core posts in the X direction of FIG. 7;

FIG. 11 shows a plot of the magnetic field in the centre of a singlecore post of FIG. 7 with DC current energisation;

FIG. 12 shows a plot of the DC magnetisation of the core of FIG. 7 withDC minor excursions about two otherwise saturated operating points;

FIG. 13 shows a plot of the relative permeability at the middle of acore post of FIG. 7 with respect to DC coil energisation and with 1,000amps of current in the 50 turn AC coil;

FIG. 14 shows a plot of the DC magnetisation of the core of FIG. 7 as afunction of the DC ampere-turns with the full AC current on the AC coilsuch that the fluxes produced by each are opposing;

FIG. 15 is an alternative form of the invention showing the same windinginterconnection and that the bottom yoke between two cores is retained;

FIG. 16 shows an arrangement of a three phase open core FCL design withthree rows and two columns of steel cores and with electricalinterconnections on each phase according to that detailed in FIG. 8;

FIG. 17 shows an alternative arrangement of the three phase open coreFCL design with two rows and three columns of steel cores and withelectrical interconnections on each phase according to that detailed inFIG. 8;

FIG. 18 shows a yoked alternative of the three phase open core FCL andwith electrical interconnections on each phase according to thatdetailed in FIG. 8;

FIG. 19 shows the experimental arrangement employed for flux density andAC steady state un-faulted insertion impedance measurements and faultcurrent limiting characterisation and with electrical interconnectionson each phase according to that detailed in FIG. 8;

FIG. 20 shows the measured un-faulted insertion impedancecharacteristics for the open core FCL experimental arrangement;

FIG. 21 shows the un-faulted steady state insertion impedancecharacteristics at different AC voltages and currents;

FIG. 22 shows Fault current characterisation plots for an open core FCLas a function of the DC bias;

FIG. 23 shows the Flux density transient characterisation plots of theopen core experimental arrangement;

FIG. 24 shows a plot of DC circuit transient voltage when the core issaturated to an extent beyond the AC coil's region of influence, andwhere the presence of the fault is detected as a slight drop in voltagebetween arrow points starting from t=0.08 seconds;

FIG. 25 shows the transient fault current plots of the experimentalarrangement with and without the open core FCL in circuit;

FIG. 26 shows the DC circuit transient current characteristics of theopen core FCL experimental arrangement;

FIG. 27 shows the Experimental arrangement of AC & DC coils for themeasurement and characterisation of the flux density, AC un-faultedinsertion impedance, and fault current limiting ability of the yoked FCLand with electrical interconnections on each phase according to thatdetailed in FIG. 8;

FIG. 28 shows the measured un-faulted steady state insertion impedanceof the open core yoked FCL experimental arrangement compared to thatmeasured on the unyoked open core FCL with limbs of the same dimensions;

FIG. 29 shows the measured un-faulted insertion impedance comparisonbetween yoked and unyoked open core arrangements and compared to variousclosed core arrangements;

FIG. 30 shows the measured un-faulted steady state insertion impedanceof the open core yoked FCL experimental arrangement compared to thatmeasured on the unyoked open core FCL with limbs of the same dimensions;

FIG. 31 shows the fault current characterisation plots for a yoked opencore FCL as a function of the DC bias;

FIG. 32 shows the flux density plot of the open core FCL experimentalarrangement, taken from a search coil around a steel limb and located atthe top of the AC coil of a yoked open core FCL;

FIG. 33 shows the DC circuit transient current characteristics of theyoked open core FCL experimental arrangement;

FIG. 34 is a schematic representation of an FCL in an electricaldistribution system;

FIG. 35 is a schematic perspective view of a single phase open core FCLin which the core includes two steel posts that are stacked end-to-end;

FIG. 36 is a top view of the FCL of FIG. 35;

FIG. 37 is a schematic perspective view of a single phase open core FCLin which the core includes a single post of pressed power;

FIG. 38 is a top view of the FCL of FIG. 37;

FIG. 39 is a schematic perspective view of a further embodiment of anFCL having a generally circular footprint and which includes yokesbetween the posts within the core; and

FIG. 40 is a schematic top view of the FCL of FIG. 39

FIG. 41 is a schematic perspective view of an FCL similar to that ofFIG. 39 sans the yokes;

FIG. 42 is a top view of the FCL of FIG. 41;

FIG. 43 is a schematic perspective view of an FCL that includes a corehaving rectangular cross-section posts arranged in a stacked 3×2 array;

FIG. 44 is a schematic perspective view of an FCL that includes a corehaving rectangular cross-section posts arranged in a side-by-side 3×2array; and

FIG. 45 is a schematic perspective view of an FCL that includes a corehaving rectangular cross-section posts arranged in a stacked 3×2 arraywhich are yoked.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While a number of embodiments are described below, further embodimentsof the invention are disclosed in Australian Patent Application No.2009901138 filed on 16 Mar. 2009 and from which priority is claimed. Thedetail of those embodiments is expressly incorporated herein by way ofcross-reference.

The following description with reference to FIGS. 1 to 6 is intended toprovide the addressee with context about the embodiments of theinvention.

Firstly, it is mentioned that frequently used parametric features of thepreferred embodiments include:

-   -   A_(core): The cross sectional area of the high permeability        cores under the AC coil    -   N_(ac): The number of AC turns,    -   N_(dc): The number of DC turns,    -   I_(dc): The DC coil current [Amps],    -   I_(ac): The AC coil current [Amps, rms]    -   f: The frequency of the electrical system    -   Z_(b): The base impedance of the electrical system that is being        protected    -   Z+: The positive sequence impedance of the system    -   I_(fp): The prospective fault current of the system    -   I_(fr): The desired reduced fault current

The fault current limiting and the insertion impedance are functions ofthe above parameters.

It will be well known to those skilled in the art that magnetisation ofa high permeability structure as required in the field of FCLs is proneto flux loss due to the following two main effects:

-   -   The fringing of the magnetic field lines around the DC bias coil        and returning through a purely air path.    -   Partial air/core flux return where the flux enters the core but        returns via an air path instead of a complete high permeability        path.

For example, an FEA analysis was conducted on the core structure shownin FIG. 1. The relevant characteristics of this core structure are:

-   -   Window dimension width=290 mm.    -   Window dimension height=350 mm,    -   Material: M6 laminated steel core.    -   Laminations employed to construct core: 0.35 mm step lapped core        structure.    -   Cross sectional area of core: 150 mm×150 mm.

Other experimental details are shown in FIG. 1 and fuller results areshown in FIG. 2.

It was found that there was a loss of magnetic flux density in the farlimbs and yokes. Table 1 below summarises the results for the FIG. 1core structure at the point of maximum flux density,

TABLE 1 Basic flux density results on prototype core of FIG. 1 Searchcoil Flux number density Location employed (T) Centre of inner limb 62.12 Inner limb close to DC coil 5 2.07 Top yoke, close to DC coil 42.01 Top yoke further from DC coil 3 2.01 Top of outer limb 2 1.96Centre of outer limb 1 1.95

The effect described here is well known to those skilled in the art. Thereduction in AC core side flux density from 2.12 Tesla to 1.95 Tesla maynot at first sight seem a disadvantage. However, it is the minor bopmeasurement on the AC coil which reveals the problem. While the DC sidecoil minor loop results in an average relative permeability of close to1.0, as expected for a saturated core, the minor loop measured at thesame level of DC coil current reveals a relative permeability of 86.This result in a high insertion impedance for the device and alsoreveals that the AC side core is not fully saturated despite observingthe classic flattening out of the B-H curve.

The approaches to reducing flux density loss and keeping the AC side ofthe core saturated include:

-   -   Employing a higher cross sectional area of core throughout the        frame.    -   Non-uniform cross sections of steel.    -   Reducing the total magnetic length of steel between the AC and        the DC coil to make a low profile core structure.

However, as an alternative to these approaches it is also practical toplace the AC coils on the near side limbs as shown in FIG. 3.

Using this technique, the flux density in the limbs immediatelyunderneath the AC coils is substantially the same as that immediatelyunderneath the DC coils.

During steady state operation, the flux from the AC coils must be suchthat the magnetic flux density in the portion of the steel core underinfluence is not de-saturated or changed substantially. For this wouldlead to higher than the minimum possible insertion impedance and causeharmonic content in the steady state un-faulted AC waveform.

During the fault limiting activity, the flux generated from the AC coilsnegates that in the steel core, de-saturating a portion of the steelcore, and causing the terminal impedance of the AC coil to rise.

In this particular arrangement, it will also be recognised that theoutside yokes and limbs are now no longer required—only the centrallimbs are needed.

The problem associated with loss of flux density in the limb containingthe AC coil is also associated with a higher steady state impedance inthe un-faulted state, also known as the insertion impedance. Theinsertion impedance associated with an AC coil is directly proportionalto the gradient of the flux density versus the magneto motive force(MMF) graph. If the portion of the core under the influence of the ACcoil is not fully saturated to a point where this slope is minimised,then the insertion impedance will be impractically high.

To illustrate the nature of insertion impedance an experimentalarrangement was constructed FIG. 4 to measure it for various locationsof the AC coil on a core with respect to the DC coil. A core and coilstructure was constructed with the details shown in Table 2 and Table 3below.

TABLE 2 No. Wire I.D. O.D. Height of No. of Resistance Dia (mm) (mm)(mm) Turns Layers (Ohms) (mm) Search Coils 15 25.5 1 1.70 0.5 DC Coil160 184 280 171 3 4.0 Former (DC) 150 160 280 Core 650 (H) × Window 450(W) mm Internal Size Core Section 100 × 100 mm

TABLE 3 Iron Core Fill Factor 0.96 Intergrating flux meter employedWalker Magnet Flux Meter Settings 25.5 × 0.96 × 100 = 2448 Copper DCCoils Used (No Superconductor) All Aluminium Construction & Support - NoMild Steel Employed Search Coils Directly Wound Tightly On Core M6Laminated Steel Core (0.35 mm thick laminations)

Reference is now made to FIG. 5. Confirmation of saturation on the DCside was made using search coils and hall probes. The use of Hall probesnecessitated the need to introduce a 1.3 mm air gap in the core whichwas not employed during insertion impedance measurements.

Other details of the experimental arrangement for the measurement ofinsertion impedance include:

DC current=100 Amps DC

AC voltage=50 V AC

Frequency of AC voltage and current: 50 Hz

AC current=28 Amps AC

AC turns=50

AC coil resistance=0.10 Ohms

FIG. 6 summarises the measured insertion impedance results. The minimuminsertion impedance is achieved with the coincident coil arrangement andwith the minimum number of ampere-turns on the DC coil required forsaturation. All other arrangements, including that where the AC coil ison the same limb as the DC coil and in close proximity to the DC coil,result in a higher insertion impedance.

Measurements of insertion impedance as a function of ampere-turns haveconfirmed that the high permeability core under the influence of the ACcoil must not only be saturated but must be “super saturated” to havethe theoretical minimum insertion impedance.

As shown in FIG. 34, the fault current limiter (FCL) is located in anelectrical distribution substation. The FCL is primarily included tolimit the fault current of a transformer, which is also illustrated.Where a substation includes more than one transformer, it is possible tohave a separate FCL for each of those transformers. However, in someembodiments, less than all of the transformers within a substation havean associated FCL.

The FCL, on the downstream side, is electrically connected to anelectrical distribution system of which the substation is a part.

In other embodiments, the transformer and the FCL are located within aninstallation other than a substation. Indicative examples include anindustrial site distribution network, between a co-generator and therest of the grid; and protecting the main electricity grid from thefault current contribution of a wind farm, wave generator,hydro-generator; or solar energy farm.

For the FIG. 34 embodiment, the power station is a coal-fired powerstation. However, in other embodiments, the power station is one or moreof a hydro-station, a nuclear power station and a wind generator powerstation.

Referring to FIG. 7 there are illustrated a series of high permeabilityposts 1 in a three phase open core FCL arrangement in accordance with anembodiment of the invention. The Z direction is defined as being alongthe longitudinal direction of the high permeability core as shown. Theposts are manufactured from transformer laminations, and the rollingdirection of the laminations is along the Z axis.

It will be appreciated that posts 1 collectively define a core for theFCL.

The high permeability posts 1 are of transformer steel laminationmaterial. In other embodiments, use is made of one or more of mild steelor other forms of magnetic steel ferrite materials or ferromagneticmaterial or granular material such as a core made from consolidatedferromagnetic powder, or a glassy amorphous core.

A DC coil 2, for the purposes of saturating a portion of the highpermeability posts 1, surrounds the complete structure outside of theenclosure. The term “surrounds” or the like are used to describe howcoil 2 encircles the enclosure or tank. That is, the DC coil extendsabout a longitudinal intermediate zone that receives the core and the ACcoil. In the illustrated embodiments, the core and the AC coil or coilsare disposed within a tank or other enclosure, and the DC coil encirclesthe enclosure. This provides for a number of packaging and performanceadvantages of the preferred embodiments. As will be mentioned below, theintermediate zones of the embodiments are defined by respective tanks.

A vessel 3 contains a dielectric insulation medium 4. This medium isalso a cooling medium for the AC coils and may be ambient atmosphericair.

There are AC coils 5 for transporting the AC current wound on insulatingformers 6 and electrically interconnected to each other in a manner suchthat the senses of the magnetic field produced by each AC coil in thecorresponding high permeability core are opposing.

There are insulation barriers 7 between phases to improve dielectricwithstand properties of the dielectric medium.

In an embodiment, the DC coil 2 is also a superconductor and morespecifically it is a high temperature superconductor housed in acryostat and cooled by a cryocooler (not shown).

FIG. 8 shows the electrical-interconnection of two AC coils in thestructure of FIG. 7 showing the sense and direction of the windingsrelative to each other.

By way of an example, the open core saturated FCL of a type shown inFIG. 7 was analysed by employing FEA. The DC and AC currents werestepped in order to find the optimum values of I_(dc) and I_(ac) a givennumber of turns on each of these windings and to understand the natureof the magnetisation of an open core. The parameters employed were thatfor a typical 15 kV class sub-station FCL and include:

-   -   Number of cores: 6    -   Length of a core post: 0.6 m    -   A_(core), the cross sectional area of each core: 0.0225 m²,        being 150 mm×150 mm in dimension    -   N_(ac): 50    -   N_(dc): 500    -   I_(dc): Stepped from zero up to 500 Amps. (Up to 250,000 DC        Ampere-turns on the DC coil)    -   I_(ac): Stepped from zero up to 1,000 Amps rms. (Up to 50,000 AC        Ampere-turns on the AC coil)

The material parameters employed are that of M6 transformer laminations,and are 0.35 mm thick.

FIG. 9 shows the distribution of magnetic field and relativepermeability across the length in the Z direction of the structure shownin FIG. 7. The region of the core suitable for placing an AC coil, thesaturated region of the high permeability core, is indicated. Thisresult shows, for example, that the AC coil should be designed such thatits height is 400 mm and situated on the core not less than 100 mm fromeither end of the core.

FIG. 10 shows a plot of the magnetic field along a line passing throughthe centre of three cores and in the X direction. This result shows thatthe magnetic field in all cores is sufficient to saturate all six coresin an X-Y array of core posts despite the non uniform distance from andgeometrical relationship with the DC coil winding.

FIG. 11 shows the DC magnetisation (I_(ac)=0) of the core in the centralregion of the core indicted in FIG. 9.

FIG. 12 Shows the Minor AC magnetisation excursion curve of the centralportion of the core at two different DC bias current values.

From a consideration of FIG. 11 alone one may draw the conclusion that aDC coil energisation of 80,000 DC ampere-turns (Equivalent to a DCcurrent of 160 Amps on the DC coil of 500 turns) would be sufficient tosaturate the core. However, a consideration of the AC coil minormagnetisation curves (FIG. 12) and the relative permeability of the coreunder AC coil energisation (FIG. 13) shows that at least 140,000 DC coilampere-turns (that is, at least 280 Amps DC on the DC coil) is requiredfor the core to have low relative permeability and therefore bestow lowinsertion impedance on the AC coil.

FIG. 12 shows that an AC current of up to 1,000 Amps on the AC coilwould de-saturate the core with a DC operating current as low as 160 A(80,000 ampere-turns). This is undesirable and such a design would leadto a high insertion impedance, high THD, and a distorted currentwaveform. By comparison, the minor DC magnetisation loop calculation atan operating point of 500 A is also shown which is a more desirableoperating point. Under these conditions the core is super-saturatedunder the AC coil and is a more suitable operating point.

In general, when considering the complete list of optimisationvariables, the combined calculations of DC magnetisation and minor DCmagnetisation is not a straight forward approach to finding suitable DCoperating Ampere-turns and requires a lengthy FEA optimisation process.To simplify the process, the inventor proposes a static magnetisationanalysis of the core with the AC coil energised to the peak of thecurrent waveform under maximum loading. FIG. 14 shows such an FEAcalculation from which it is clear that in this case a DC magnetisationof 150,000 Ampere-turns are required for the core to remain insaturation at each and every instantaneous point of the AC currentwaveform.

It is important practically for a fault current limiter to have a lowinsertion impedance. In the present embodiment this is achieved byensuring that the volume of the steel core under the direct magneticinfluence by the AC coil is fully saturated by the DC coil to a level,B_(sat), such that it remains saturated in the normal AC steady stateoperating condition.

The saturable core FCL design shown in FIG. 7 meets the four maincriteria for a FCL and has the advantages of:

Lower mass through the absence of the yokes and outer limbs.

Lower footprint for a given fault current and steady state rating.

Economic cost of construction.

By inverting the relative locations of the AC and DC coil, the followingtechnical benefits are also obtained:

-   -   The structure becomes directly amenable to high voltage and        extra-high voltage designs without requiring special dielectric        feed-throughs or vacuum-to-dielectric interfaces. The central        part of the high permeability core may be immersed in liquid or        gaseous dielectric fluid in much the same way that a power        transformer is completely immersed in dielectric fluid.    -   Aspects of the technology and body of knowledge about high        voltage transformer design with synthetic silicon oil or other        dielectrics are applicable to this basic design including        gaseous high voltage dielectrics such as SF₆. This reduces a        substantial risk involved in the design and development process        for high voltage versions of these devices.    -   Standard well-known solid materials used for immersion in liquid        dielectrics and employed at high static voltages may be        employed.    -   The AC phase coils envelope an area of the steel limbs which is        super saturated.    -   The extent of electromagnetic influence of the AC coils are such        that the insertion impedance is very close the theoretical        minimum that it can be. For example, as illustrated in FIG. 9        and FIG. 13. In these figures the FEA has revealed that the        relative permeability of the cores is very close to unity        despite the non-uniform distance from the race track DC coil.

In another embodiment, the open cores are tapered to the ends in amanner which keeps all of the core saturated.

In the further embodiment shown in FIG. 15 the core posts of each phaseare connected with a yoke but remain open at one end.

FIG. 19 shows a FCL having a single phase open core with the followingdetails:

Core dimensions: 100 mm×100 mm×570 mm

Number of turns on each AC coil core: 20

Number of turns on DC biasing coil: 100

The results from the experimental arrangement shown in FIG. 19 are givenin FIGS. 20 to 26. More particularly, FIG. 20 shows the measured steadystate un-faulted insertion impedance at 50 Hz across the open core FCLterminals. There is distinct change in the characteristic of theinsertion impedance when sufficient DC bias is applied. In part A ofFIG. 20, below the minimum insertion impedance, the magnetic saturationof the high permeability core has not yet reached the complete volume ofthe core under the magnetic influence of the AC coil. Hence, themeasured insertion impedance is high.

In part B of FIG. 20 the magnetic saturation of the high permeabilitycore has reached the extent of the AC coil's influence. This shows thata region of the high permeability core equal to at least the height ofAC coil must be saturated by the DC coil in order to obtain the minimuminsertion impedance for the open core design.

FIG. 21 shows the un-faulted steady state insertion impedancecharacteristics of the open core FCL for a number of different voltageand current levels and shows that this quantity is independent of the ACvoltage level and current level.

The transient AC current plots in FIG. 22 display the difference in thefault current with and without the FCL placed in the measurementcircuit. This data shows that significant reductions in fault currentare possible for the open core FCL arrangement.

FIG. 23 shows the measured flux density in the steel core as a functionof time during the fault current event. The fault current effectivelyde-saturates the steel core region under AC coils. This results in theFCL having a high impedance during the fault and hence effective,intrinsic fault current limiting properties.

The data shown in FIG. 24 indicates that if the high permeability coreis sufficiently saturated that the transient voltage induced into the DCcoil remains manageable and not unduly deleterious during the fault.This is analogous to the classic saturated FCL core design.

FIG. 25 shows the measured transient fault current waveforms with thecalculated prospective fault current after allowing for the AC coilresistance and the steady state un-faulted inductive component of theFCL AC coil impedance. The additional reduction in fault current from apeak of 2,000 Amps to a peak of 1,100 Amps is due to the additionalchange in magnetisation after allowance for AC coil resistance and thesteady state un-faulted insertion impedance.

FIG. 26 shows the measured DC current transients during the fault eventat a number of different DC bias current values. The inducted transientDC current is insignificant if the steel core is sufficiently biased.

FIG. 27 shows an alternative experimental arrangement of the open coreFCL which includes yokes between the cores and is designed to decreasethe DC bias ampere-turns required for low insertion impedance. Detailsof the design as are follows:

High permeability core dimensions: 100 mm×100 mm×570 mm (High)

Yoke dimensions: 100 mm×100 mm×250 mm (High)

Number of turns on each AC coil core: 20

Number of turns on DC biasing coil: 100

A comparison between the insertion impedance results obtained for theyoked and unyoked configurations is given in FIG. 28 where the measured50. Hz un-faulted steady state insertion impedance characteristics of anopen core FCL with and without yokes is shown.

FIG. 29 shows that yoking of the core arrangement within the DC biascoil shifts the magnetization curve to the left, allowing lessampere-turns to be used to obtain minimum insertion impedance.

FIG. 30 shows the full range of insertion impedance for the yokedconfiguration, which shows significant improvement in the faultimpedance of this arrangement at lower DC applied ampere-turns.

Fault current plots for the yoked open core FCL experimental arrangementin FIG. 31 show the difference the presence of the yoked FCL makes forvarious DC biasing modes in comparison to a system without the FCL.

The magnetic flux density in the highly permeable core material measuredat the top of the AC coil was also measured in FIG. 32 indicating thesame characteristic behaviour as in the non yoked open core experimentalarrangement.

FIG. 33 shows the DC circuit transient current waveforms across a rangeof different bias levels. As for the unyoked open core FCL arrangement,the induced transient DC current is insignificant for sufficientlybiased cores.

The primary benefit of arranging the DC and AC coils as illustrated inthe embodiments is that the AC coils experience the full DC flux densityof the steel core under the DC coil. Classic saturated FCL designssuffer from the disadvantage of transporting the flux from the DC limbsto the AC limbs through the upper and lower yokes and around the miteredjoints within the core. The present embodiments dispenses with the yokeand the AC side limbs making flux transport from the DC to the AC coilsalmost 100% efficient.

It will be appreciated that in the illustrated embodiments each faultcurrent limiter includes at least one input terminal in the form of ahigh voltage bushing for electrically connecting to a power source, suchas a transformer, that provides a load current. Each of the embodimentsalso includes at least one output terminal, also in the form of one ormore high voltage bushings, for electrically connecting with a loadcircuit, such as an electrical distribution system, that draws the loadcurrent. Also included is a magnetically saturable core and at least oneAC coil—typically one coil for each phase of the load current—that iswound about a longitudinal portion of the core for carrying the loadcurrent between the input terminal or terminals and the output terminalor terminals. A DC coil induces a magnetic field in at least the portionof the core and extends about a longitudinal intermediate zone thatreceives the core and the AC coil. In the illustrated embodiments, theintermediate zones are defined by respective tanks. The field induced bythe DC coil magnetically biases the core such that the AC coil movesfrom a low impedance state to a high impedance state in response to oneor more characteristics of the load current.

It will be appreciated that in many applications, particularly where anFCL is to be retro-fitted to an existing facility, the physical spaceavailable to accommodate the FCL is often limited. Even more usually,the most significant physical constraint is the footprint available forthe FCL. Reference is now made to FIGS. 35 and 36 where there isillustrated a single phase open core FCL that has been developed forsmall footprint applications. The FCL includes an input terminal in theform of a high voltage bushing for electrically connecting to a powersource (not shown) that provides a load current. An output terminal, inthe form of a further high voltage bushing, electrically connects with aload circuit (not shown) that draws the load current. A magneticallysaturable core has the form of two like high permeability laminatedsteel posts that extend longitudinally and which are stacked with eachother end to end. An AC coil has two coil segments that are oppositelywound about respective longitudinal portions of the posts for carryingthe load current between the input terminal and the output terminal. ADC coil, in the form of two spaced apart sub-coils, induces a magneticfield in at least the portions of the posts and extends about alongitudinal intermediate zone that receives the core and the AC coil.The zone, in this embodiment, is defined by the tank. The fieldmagnetically biases the posts such that the AC coil moves from a lowimpedance state to a high impedance state in response to one or morecharacteristics of the load current.

A further small footprint embodiment is illustrated in FIGS. 37 and 38.In this embodiment, use is made of a pressed power core. This provides ahigher fill factor of high permeability material within thecross-sectional area of the AC coil than is able to be achieved withlaminations. Accordingly, for the same footprint, and assuming all elseis equal, the FCL of this embodiment provides improved performance overthan of FIGS. 35 and 36.

In a further embodiment, the FCL of FIGS. 37 and 38 is developed toprovide the same performance as the FCL of FIGS. 35 and 36. Due to thehigher fill factor, this further embodiment has a smaller footprint thanthe FCL of FIGS. 37 and 38.

Another embodiment of the FCL is illustrated in FIGS. 39 and 40. Thisembodiment is a three phase open core FCL having three pairs of paralleland longitudinally coextensive posts—one pair of posts for eachphase—for collectively defining the core. The posts have a constant anduniform transverse cross-section that is asymmetric. The pairs of postsinclude yokes, and the posts, the associated AC coils and the yokes areall disposed within a tank containing a dielectric medium that also actsas a cooling medium.

FIGS. 41 and 42 illustrate a further embodiment that is similar to thatof FIGS. 39 and 40, with the major difference being the omission ofyokes to further reduce the amount of volume occupied by the FCL.

It will be appreciated that the fault current limiters illustrated inFIGS. 39 to 42 include like posts having asymmetric posts that arearranged relative to each other to define generally a cylinder. Thisshape and relative arrangement or relative orientation of the posts alsocontributes to a small footprint for the FCL.

In other embodiments different approaches are taken to optimize thefootprint for the FCL, or to otherwise address any accommodationspecifications for a given site. For example, reference is made to FIG.43 that illustrates an FCL that includes a core having rectangularcross-section posts arranged in a stacked 3×2 array. The two coilsegments for the AC coil of the same phase are arranged one under theother. This configuration of open core FCL is used, for example, wherethe footprint of a site is limited, and a greater height is permitted.

A further embodiment is illustrated in FIG. 44 where an FCL includes acore having rectangular cross-section posts arranged in a side-by-side3×2 array. This configuration of open core FCL is used, for example,where the height requirements are limited, but a greater footprint ispermitted.

A further example of an FCL is illustrated in FIG. 45 that includes acore having rectangular cross-section posts arranged in a stacked 3×2array which are yoked.

Compared to the known picture frame style “closed core” saturable corefault current limiter, the above described embodiments have thefollowing advantages:

-   -   A significant reduction in the mass of steel required and hence        reduced cost of manufacture, transportation, and site location.    -   For similar performance, a reduction in the footprint. This is        particularly advantageous in easing placement issues at dense        urban locations.    -   In those cases where a superconductor is employed for the DC        bias coil or coils, a lower cryostat surface area. This results        in less steady state ambient heat loss, and hence a lower        cryocooler power requirement.    -   Mechanical de-coupling of the DC bias coil and cryostat from the        AC phase coils and steel core. This allows the oil tank to be        lowered into the DC coil warm bore area, or the DC coils may be        lowered over the oil tanks containing the phase coils and cores.

Compared to the alternative fault current limiter arrangements such asresistive types, resistive types with external or internal reactor,shielded core, solid state, the saturable open core fault currentlimiter has these advantages:

-   -   The open core fault current limiter will not do harm to a        protected line and does not need to be isolated from a protected        line if any aspect of the superconducting portion fails, whether        this be the DC coil, vacuum system, or cryogenic system. Hence,        the open core fault current limiters of the embodiments are        inherently fail safe and are able to be left in the protected        line under these conditions. Moreover, the redundancy associated        with alarms and detection of internal faults is able to be much        less stringent compared to designs which must be switched out of        service for an internal fault.    -   None of the DC bias coils (whether it be a superconducting coil        or otherwise) is directly connected to the high voltage or high        current line of the grid or electricity supply that is being        protected. Hence, simple, established and well known dielectric        design procedures are able to be used to design the high voltage        portion.    -   Liquid cryogens are not used as an AC dielectric and, hence,        issues associated with these liquids do not exist in the design        of the preferred embodiments,    -   Superconducting elements are not stressed by the fault current.        Accordingly, there is very little induction of current and        voltage into the DC coil during a fault.    -   The superconductor does not quench during a fault and hence is        able to be used in-line where auto re-closers or re-closing        logic is employed on the breakers and isolators of a protected        line.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the above description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, Figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby expressly incorporated into the description of theinvention, with each claim standing on its own as a separate embodimentof this invention.

Further embodiments of the invention are disclosed in Australian PatentApplication No. 2009901138 filed on 16 Mar. 2009 and from which priorityis claimed. The detail of those embodiments is expressly incorporatedherein by way of cross-reference.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments—including those embodiments disclosedin the patent specifications from which priority benefit is claimed—aremeant to be within the scope of the invention, and form differentembodiments, as would be understood by those skilled in the art. Forexample, in the following claims, any of the claimed embodiments can beused in any combination.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

Those skilled in the art will recognise that these are examples appliedto specific designs that were manufactured and that detailed results forother designs with different construction details will differ. The mainconclusions and pattern of results are to be considered.

Although the invention has been described with reference to specificexamples it will be appreciated by those skilled in the art that it maybe embodied in many other forms.

The invention claimed is:
 1. A fault current limiter, comprising: aninput terminal electrically connecting to a power source that provides aload current; an output terminal electrically connecting with a loadcircuit that draws the load current; a magnetically saturable core; anAC coil wound about a longitudinal portion of the core for carrying theload current between the input terminal and the output terminal; anenclosure for housing the magnetically saturable core and AC coil,wherein the enclosure includes a cooling means in addition to adielectric material; and at least one DC coil for inducing a magneticfield in at least the portion of the core and extending about alongitudinal intermediate zone that receives the core and the AC coil,wherein the field magnetically biases the core such that the AC coilmoves from a low impedance state to a high impedance state in responseto one or more characteristics of the load current.
 2. A fault currentlimiter according to claim 1, wherein the limiter includes at least twoDC coils.
 3. A fault current limiter according to claim 2, wherein theDC coils are spaced apart.
 4. A fault current limiter according to claim1, wherein, in the low impedance state, the portion is magneticallysaturated.
 5. A fault current limiter according to claim 4, wherein, inthe low impedance state, the core is magnetically saturatedlongitudinally beyond the portion.
 6. A fault current limiter accordingto claim 1, wherein, in the high impedance state, the portion is out ofmagnetic saturation.
 7. A fault current limiter according to claim 1,wherein in the low impedance state, the impedance of the AC coil issubstantially equal to the theoretical air core impedance of the ACcoil.
 8. A fault current limiter according to claim 1, wherein the DCcoil includes a high conductivity material.
 9. A fault current limiteraccording to claim 8, wherein the high conductivity material is copper.10. A fault current limiter according to claim 8, wherein the highconductivity material is aluminium.
 11. A fault current limiteraccording to claim 8, wherein the high conductivity material is a hightemperature superconductive material.
 12. A fault current limiteraccording to claim 8, wherein the high conductivity material is a lowtemperature superconductive material.
 13. A fault current limiteraccording to claim 1, wherein the DC coil is disposed outside of andsurrounding the enclosure.
 14. A fault current limiter according toclaim 1, wherein the dielectric material is in the form of a liquid or agas.
 15. A fault current limiter according to claim 1, wherein the ACcoil extends longitudinally beyond the DC coil.
 16. A fault currentlimiter accordingly to claim 1, wherein the DC coil is in an open-corearrangement with the AC coil.